Investors Brace For Volatility After Friday Stock Market Sell-Off

September is the worst month of the year for stocks

Investors just got an unwelcome wake-up call.

After snoozing through most of August, the stock market roared back to life Friday as major indexes witnessed their worst plunge since the days following Brexit.

Following the S&P 500's drop through key support levels, analysts expect more downside pressure in the week ahead as investors brace for the possibility that interest rates could be on the rise sooner than later.

"The weakness could continue given the stock market's dependence on central bank generosity," said Kristina Hooper, U.S. investment strategist at Allianz Global Investors.

The S&P 500 closed at a two-month low, tumbling 53.49 points, or 2.5%, to 2,127.81. The Dow Jones Industrial Average lost nearly 400 points, finishing at 18,085.45 and the Nasdaq Composite Index skidded 133.57 points, or 2.5%, to end at 5,125.91. All three benchmarks shed more than 2% for the week.

Read:Nasty correction is the best-case scenario for the bulls (http://www.marketwatch.com/story/nasty-correction-is-the-best-case-scenario-for-the-bulls-2016-09-09)

The selloff was sparked by comments from Boston Fed President Eric Rosengren who suggested that the Federal Reserve should resume raising interest rates soon (http://www.marketwatch.com/story/feds-rosengren-backs-interest-rate-hike-2016-09-09), rather than waiting and risk missing its timing.

Expectations for a rate increase had risen following comments by Fed Chairwoman Janet Yellen last month that the case for a tighter monetary policy is strengthening although she did not indicate a specific timetable. Those expectations subsequently fluctuated in the wake of mixed economic data.

But the fact that the European Central Bank did not extend its current asset-buying program beyond March 2017 at its Thursday policy meeting also fueled speculation that the Fed will have more room to maneuver.

"Expectations that the Fed could raise interest rates this month are on the upswing again following Thursday's ECB meeting where it apparently didn't discuss new stimulus--a shift from dovish to neutral that has opened the door for the Fed to raise interest rates soon," said Colin Cieszynski, chief market strategist at CMC Markets.

Market participants have argued the Fed would have to be more circumspect about tightening its policy if its counterparts, including the ECB and the Bank of Japan, continue to loosen their policies.

"Monday could be the Fed doves' last stand with Gov. Lael Brainard speaking, one of the few openly dovish members left along with [James] Bullard after New York Fed President Dudley switched sides last month," Cieszynski said.

See:Clinton Treasury secretary contender could send key signal about crucial Fed meeting (http://www.marketwatch.com/story/clinton-treasury-secretary-contender-is-key-vote-at-crucial-fed-meeting-2016-09-01)

Corporate earnings will take a back seat for now with no major S&P 500 company slated to announce financial results. But volatility could pick up given that Friday marked the first time since July 8 that the S&P 500 saw a daily move of more than 1%. The CBOE Market Volatility Index , which measures Wall Street's level of fear, also surged 34% to above 16, its biggest one-day move since late June (http://www.marketwatch.com/story/wall-streets-fear-gauge-jumped-the-most-since-brexit-2016-09-09).

While neither of those factors indicate any sort of panic, it does suggest that investors may need to get used to wider price swings.

"The month of September has historically exhibited elevated volatility in both directions. In other words, while today's action is a clear change in character, two way volatility has a high probability of persisting over the coming weeks." said Frank Cappelleri, executive director at Instinet LLC.

Read:Volatile September may have investors pining for dog days of August (http://www.marketwatch.com/story/volatile-september-may-have-investors-pining-for-dog-days-of-august-2016-08-30)

After the S&P 500 crumbled through 2,135 without significant support (http://www.marketwatch.com/story/the-stock-market-rout-just-drove-sp-500-dow-below-a-key-level-2016-09-09), technical strategists are now looking to the index's 200-day moving average of 2,057 as the next level to defend.

"Both the Dow and S&P broke their 50-day averages, signaling that support has caved. September is historically the worst month of the year for stocks and it looks like a seasonal decline may be getting under way," Cyzneski said.

The is also packed next week which will provide more fodder for a market starved of catalysts. Among the economic data on tap are weekly jobless claims, monthly retail sales, industrial output and the Consumer Price Index.

"I expect the averages to hinge upon the infinite interpretations of each data point, drawing broad based conclusion from each and every release leading into the following week's FOMC meeting," said Kent Engelke, chief economic strategist at Capitol Securities Management Inc.

The Federal Open Market Committee will meet Sept. 20-21.